Field of the Invention
The present invention relates to the field of piezoelectric ceramic compositions, methods of making them and devices comprising them. More particularly, embodiments of the present invention relate to piezoelectric ceramic compositions, which can be incorporated into actuators, transducers, transformers, sensors, and energy harvesters. Particular embodiments of the invention include layered ceramic materials with texture and improved piezoelectric and/or magnetostrictive properties.
Description of Related Art
Piezoceramic materials based on lead zirconate and lead titanate, otherwise referred to as lead-zirconate-titanate ceramics (PZT), are useful in a number of industries as actuators and transducers due to their good mechanical and piezoelectric properties. In an attempt to improve on these properties and increase performance and efficiency of the ceramic materials, PZT is sometimes doped with alkaline earth metals or rare earth metals. Doping, however, has its limitations in that it is difficult and rare to achieve a level of performance equivalent to that of the giant piezoelectric properties of single crystals.
For example, oriented relaxor-based piezoelectric single crystals such as Pb(Mg1/3Nb2/3)O3—PbTiO3 (PMN-PT) and Pb(Zn1/3Nb2/3)O3—PbTiO3 (PZN-PT) with rhombohedral structure near morphotropic phase boundary (MPB) exhibit d33 of >2000 pC/N and k33 of >92%. Such giant piezoelectric properties of rhombohedral crystals are related to their piezoelectric anisotropy and engineered domain state that facilitate the rotation of polarization toward the direction. Owing to their ultrahigh d and k values, domain-engineered piezoelectric crystals are considered very promising materials for actuators, transformers, sensors, and recent emerging energy harvesters. Single crystals usually involve a costly fabrication process (high-temperature melting and annealing process for a long time and use of platinum crucibles) as compared to a polycrystalline ceramic synthesis process. Additionally, the size of traditional crystal products restricts their industrial applications especially where large dimension components are required, such as in a low frequency resonance mode vibration harvester.
Thus, there is a need for developing polycrystalline ceramic products with properties similar to that of their single crystal counterparts. Processes exist for preparing polycrystalline species, however, such processes typically result in synthesizing randomly oriented polycrystalline ceramics. Although preparing randomly oriented polycrystalline products is economical and not limited in product size, the superior piezoelectric properties of single crystals are hardly observed in randomly oriented polycrystalline ceramics where piezoelectric properties are averaged out by randomly distributed crystalline grains.
One method of improving on the piezoelectric properties of these ceramic materials involves texturing the PZT ceramic. Textured ceramics have grains or crystallites in the ceramic structure with the same orientation as contrasted with randomly oriented structures. Such an ordered orientation leads to improved properties of the material.
Thus, what is needed is a strategy for overcoming the drawbacks of single crystals and achieving high piezoelectric properties in a polycrystalline structure. More particularly, what is needed is a low cost method for producing piezoelectric ceramic materials with high d and/or k values and with texturing (grain orientation along the crystallographic direction) that have an engineered domain state similar to that of single crystals.